In recent years, there has been an increase in the number of food products that are designed or intended to go directly from the freezer to the microwave for heating or cooking. Such food products often require packaging that also can go directly from freezer storage temperatures, typically temperatures ranging from about −20° C. to about 0° C., to microwave cooking temperatures ranging from about 71° C. or greater. Additionally, it is desirable that such packaging perform without fracturing in the freezer, and without melting or bursting in the microwave. With respect to foods such as vegetables, the temperatures reached during microwave cooking typically range from about 71° C. to about 105° C. However, when the packaged food items include meats, fish, grease, and/or oils, for example, the temperature of the items can reach up to about 150° C. when heated during microwave preparation.
One type of container typically used for packaging frozen microwaveable foods is a rigid container, such as a paperboard carton or container structure, coupled with a lid. Such rigid microwaveable containers are disclosed, for example, in U.S. Pat. No. 6,066,375, to Shanton; U.S. Pat. No. 6,379,497, to Sandstrom, et al.; and U.S. Pat. No. 4,836,438, to Rigley. The material and structure of such containers are not suitable for use on automatic vertical-form-fill-seal machines to package frozen foods, and tends to result in increased production costs.
Additionally, there are a variety of multilayer films in the art that can be used as packaging for frozen food products. Such multilayer films are primarily made from polyethylene resins which allegedly exhibit cold-temperature resistance, i.e., being able to withstand temperatures ranging from about −20° C. to about 0° C. without shattering or fracturing. Polyethylene films, however, soften at microwave temperatures, typically temperatures ranging from about 160° F. to about 220° F. (about 71° C. to about 105° C.) or greater, which causes weakening of the bonding strength of the polyethylene seal layer and/or between the other polyethylene film layers and adjacent layers, leading to seal failure and/or layer delamination and leakage of the package contents during microwaving. Thus, frozen food products must be removed from such food packaging materials and then placed on a microwaveable dish or container before the products can be heated or cooked in a microwave. This conventional microwave cooking method tends to result in large temperature variation in the foods and can cause the formation of cold, hot and/or dried spots in the foods due to excessive moisture loss in certain areas of the food. Such additional food preparation steps also require additional time, and tend to be messy.
There are also microwaveable film packaging materials known in the art. One such packaging material is a three-layer laminated film structure containing polypropylene homopolymer, adhesive, and polyethylene terephthalate (PET). However, this particular film packaging material in the art becomes brittle when subjected to freezer temperatures and breaks apart or shatters. Thus, although the packaging material may be microwaveable, it is not suitable for frozen food applications.
To meet the requirements for freezable/microwaveable food packaging, the packaging materials must have two essential properties: (1) they must have good low temperature (subzero) resistance so that the packaging can withstand subzero freezer storage temperatures without fracturing or shattering; and (2) they must have good high-temperature resistance to prevent the package from melting or bursting in the microwave. Currently available film packaging materials can only handle one end of the temperature spectrum or the other, not both, and therefore are unsuitable for the storage of food products in the freezer which will subsequently be subjected to heating or cooking in the microwave to prepare the food products contained therein for consumption.
Thus, there is a need in the art for films and packaging materials that are designed to package frozen foods for subsequent microwave heating or cooking without requiring removal from the packaging. There is also a need for freezable, microwavable films and packaging materials that reduce the amount of moisture loss by foods during microwave cooking. There is a further need for freezable, microwavable films and packaging that reduce the temperature variation that results during microwaving of foods.